There is an oil galley which leads up from the main galley/oil pump region and supplies oil to those parts of the engine which are above the head gasket. The items that must be supplied with oil are the cam journals, the lifters and valve springs and of course the timing chain tensioner. But because these items require a much smaller volume of oil than the main/rod bearings, there is a restrictor orifice pressed into the vertical oil galley right at the deck surface (the top of the block). It is near the front on the driver's side.

The photo shows the restrictor, which has been removed from the block and turned upside down so that the orifice can be seen. That small hole supplies ALL of the oil to the top of your engine! Imagine how easy it is for it to become clogged with debris.

Here is the restrictor in its upright position. It is pressed into a large recess in the block deck right where the vertical oil galley comes up. So the oil has to squeeze its way through the small orifice before it can make its way to the head. If you are rebuilding your engine it is a good idea to remove the restrictor before sending your block to the machine shop. First, there may well be debris caught behind the restrictor which can only be cleaned out with the orifice removed. Second, your machinist may remove the restrictor and forget to replace it. If you do not notice this and proceed to assemble your engine you will experience low oil supply to your main and rod bearings and your engine will probably no last very long. Note the threads in the top of the restrictor. These are used to "pull" the restrictor out of the block.

You can pull the restrictor from the block using a long bolt and a nut along with some washers to use as spacers. The basic idea is to screw the bolt into the restrictor until it bottoms. A nut on the bolt can then be turned in the downwards direction. This has the effect of pulling the restrictor up. You will have to play around with this a bit but it is not too tough, mine came right out and so did Jim Butterworth's. Make sure you don't loose it during the rebuild and for God's sake don't forget to reinstall it. I used some Loctite Sleeve Retainer but this is probably superfluous as the restrictor has nowhere to go with the head in place. I do not remember the thread size of the restrictor - either 4 mm or 6 mm - but the astute observer will notice the 10 mm wrench in the picture.